Storage systems may include a plurality of storage devices for storing data as well as storage controllers for controlling the storage devices, and aim to provide large-capacity data storage spaces in the host computer.
Storage systems are required to store large amounts of data at low cost. As a technique for satisfying this requirement, for example, a technique called Thin Provisioning disclosed in Patent Document 1 is known. Storage systems utilizing the thin provisioning technology may provide a virtual volume to the host computer. In an initial state, no storage area is allocated to the storage space of the virtual volume. Upon receiving the write request from the host computer, the storage system allocates a storage area to the area that received the write request. The total size of the virtual volumes may be larger than the total amount of the storage areas mounted in the storage system. In thin provisioning technology, it is not always necessary to prepare a storage area corresponding to the size of the virtual volume from the beginning, such that that storage area can be saved.
In storage systems that utilize thin provisioning technology, the storage area to be allocated to the virtual volume is stored in a pool and managed. However, if all the storage areas belonging to the pool are allocated to a virtual volume, the storage system cannot receive write requests for the unallocated area from the host. According to the technology disclosed in Patent Document 1, the storage system monitors the free capacity of the pool so as to prevent such a situation from occurring. When the usage amount of the whole pool (the amount of storage area allocated to the virtual volume) exceeds the threshold value, the storage system adds a new storage area to the pool.
Another technique for cost reduction is compression technology. For example, Patent Document 2 discloses a storage system equipped with a storage device (flash package) having a compression function. In this storage system, write data from a host computer is compressed and stored in the flash package. Since the size of the data stored in the flash package is reduced by compression, the storage capacity (apparent storage capacity) that the flash package provides to the storage system may be larger than the total size of the storage medium (flash memory ships) included in the flash package.